Field of the Invention and Related Art Statement
This invention relates to an optical information recording and reproducing apparatus wherein the offset of such displaced infromation signal as focusing error signal or tracking error signal in recording can be eliminated.
Recently, there has come to be noted an optical information recording and reproducing apparatus wherein, by condensing a light beam and radiating it onto an optical recording medium, the information can be recorded at a high density in this recording medium and, by receiving the light returning from this recording medium with a light detector, the recorded information written in the recording medium can be read out (reproduced) at a high speed.
In the above mentioned apparatus, as the recording or reproduction is made at a high density, the light beam condensed and radiated onto the recording medium must be held in a focused state and on-track state. Therefore, the above mentioned apparatus is usually provided with a focusing controlling means and tracking (radial) controlling means. By these controlling means, the focusing and radial displaced informations included in the light returning from the above mentioned recording medium are detected as a focusing error signal and tracking error signal and the light beam is held in a focused state and on-track state on the basis of these error signals.
Various systems for detecting the above mentioned focusing error signal and tracking error signal have been suggested. A prior art example of an optical information recording and reproducing apparatus wherein a critical angle method is used for a focusing error signal detecting system is shown in FIG. 1.
As shown in this drawing, the optical information recording and reproducing apparatus is provided with an optical pickup 20 arranged as opposed to the surface of a disc-like recording medium (which shall be mentioned as a disc hereinafter) 6. This optical pickup 20 can be moved by a moving means not illustrated in a direction crossing a recording track in the above mentioned disc 6 rotated and driven, for example, in the direction indicated by the arrow T.
A laser diode 1 as a light source is contained within a housing of the above mentioned optical pickup 20. A diffused light, for example, of a P-polarization emitted from this laser diode 1 is made of a parallel light bundle by a coupling lens 2. This parallel light bundle is incident upon a polarized beam splitter 3, is passed substantially by 100%, is circularly polarized by a .lambda./4 plate 4 and is then condensed and radiated onto the above mentioned disc by an objective lens 5. The light bundle condensed and radiated onto this disc 6 is radiated in a spot-like nearly focused state onto a recording layer of this disc 6. The reflected light from the recording layer of this disc 6 is condensed to be a substantially parallel light bundle by the above mentioned objective lens 5, is made an S-polarization of a polarizing direction different by 90 degrees from that in the going path by the above mentioned .lambda./4 plate 4 and is incident upon the above mentioned polarized light bundle splitter 3. The reflected light from this disc 6 is reflected substantially by 100% by the above mentioned polarized beam splitter 3, is further reflected by a reflecting prism 7 and is incident upon a critical angle prism 8. The light bundle reflected by the slope of this critical angle prism 8 is received by a light detector 9 provided in the position of receiving a diffracted light of a far field. This light detector 9 is formed of such light receiving device as, for example, a 4-divided photodiode. A difference signal A-B is obtained by such operational circuit 10 as a differential amplifier from the respective outputs A and B of light receiving devices 9A and 9B adjacent to each other in the horizontal direction in FIG. 1. A focusing error signal S.sub.FE is produced by this difference signal A-B. On the other hand, a tracking error signal is produced by the difference signal between the light receiving devices adjacent to each other in the direction vertical to the paper surface of FIG. 1.
The above mentioned focusing error signal S.sub.FE is applied to a focusing coil 13a of a lens actuator 13 through a phase compensating circuit 11 and objective lens driving circuit 12. The above mentioned objective lens 5 is moved in the direction vertical to the disc 6 surface by the above mentioned lens actuator 13 on the basis of the above mentioned focusing error signal S.sub.FE so that the focusing may be controlled.
Also, the above mentioned tracking error signal is applied to a tracking coil not illustrated of the above mentioned lens actuator 13 through a phase compensating circuit and objective lens driving circuit not illustrated so that the spot light condensed and radiated by the objective lens 5 may follow a predetermined track.
Also, a data signal is obtained from the sum signal of all the light receiving devices of the above light detector 9.
Now, in the apparatus wherein the information is recorded or reproduced by such structure variation and pits as vary the reflection rate in the recording medium as a recording form, the above mentioned focusing error signal S.sub.FE will offset in some case. The displacement of the optical pickup 21 forming member of the light detector 9 or the like or the offset or the like caused in the signal processing circuit of the focusing error signal S.sub.FE is considered for this cause. If the displacement and offset of the signal processing circuit are adjusted so as to be eliminated at the time of the reproduction, the offset at the time of the recording will be able to be dissolved.
As different from this offset, there is an offset not produced at the time of the reproduction but produced at the recording time.
That is to say, at the time of the recording, in response to the recorded data, the light beam will be set at a light emitting power high in the energy density in the pulses. Pits 21 or the like will be formed as shown in FIG. 2 in the land part of the recording medium onto which the light beam is condensed and radiated with this light emitting power. These pits 21 will not be produced uniformly for the entire beam spot 22 scanned at a high speed on the recording medium but will be produced earlier in the part of a larger radiating energy. Therefore, as shown in FIG. 2, an unrecorded part 23 and recorded part 24 will exist simultaneously in the rotating direction (direction tangential to the track) indicated by the arrow T within the beam spot 22 on the recording medium. Therefore, as a result of the diffraction effect by the reflection rate difference and phase difference between the above mentioned unrecorded part 23 and recorded part 24, the beam distribution in the tangential direction T of the far field pattern 25 of the reflected light from the above mentioned recording medium will become non-uniform. As a result, an offset will be produced in the above mentioned focusing error signal S.sub.FE between the reproducing section and recording section. The manner of the variation of this focusing error signal S.sub.FE is shown in FIG. 3.
The optical adjustment is so made that the offset may be 0 at the time of the reproduction and the reproducing state R and recording state W are changed over to each other according to such recording instructing pulses as are shown in (a). Just after the reproducing state R is changed over to the recording state W, as shown in (b), in the focusing error signal S.sub.FE, as described above, the beam distribution in the tangential direction T of the far field pattern 25 of the reflected light from the recording medium will be non-uniform and therefore an offset OS will be produced and the focusing error signal S.sub.FE will gradually converge toward the ground level GND while being swung by the excess response of the servo system. By the way, in the drawing, TR represents the excess response section of the servo system. The above mentioned focusing error signal S.sub.FE will be greatly swung just after the reproducing state R is changed over to the recording state W but will be little swung just after the recording state W is changed over the reproducing state R, because the non-uniformity of the beam distribution in the tangential direction T of the far field pattern 25 of the above mentioned reflected light and the variation by the defocused amount of the non-uniform degree will occur only during the recording.
Thus, in the conventional optical information recording and reproducing apparatus, an offset will be produced in the focusing error signal S.sub.FE in the reproducing state R and recording state W and therefore it has been difficult to make the focusing control most suitable to both states.
If an offset is produced in the above mentioned focusing error signal S.sub.FE, the light spot 22 particularly at the time of recording will expand, the radiating power will be short and the data will not be able to be written in in the normal state. Therefore, there will be produced a problem that the reliability of the optical information recording and reproducing apparatus will be reduced.
By the way, the above mentioned offset will be produced not only in the focusing error signal obtained by the critical angle method but also in the focusing error signal by another system and the tracking error signal depending on the system.